<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>rem基本使用</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }
    /* 1.给Html标签加字号 */
    html {
      font-size: 30px;
    }

    /* 2.使用rem单位书写尺寸 */
    .box {
      width: 5rem;
      height: 3rem;
      background-color: pink;
    }
  </style>
</head>
<body>
  <div class="box"></div>
</body>
</html>